The crop-gathering header of a harvester combine travels as close to the surface of the ground as possible in order to gather and harvest low-lying feed crops such as soybeans, and consequently the cutting mechanism carried by the header frequently encounters surface rises and depressions in the contour of the terrain which cause the header to bounce and occasionally dig the crop-cutting mechanism into the ground. The header should be maintained at a minimum position above the ground in order to avoid picking up stones and running the cutting mechanism into the ground, but in most self-propelled combines the operator's platform is in a cab above the crop-gathering header from where it is difficult for the operator to tell how high the header is above the ground. In order to relieve the operator of this task, header height controls are known for the purpose of detecting ground contour variations and signaling them to hydraulically or electrically operated power devices which then adjust the height of the header above the ground, sufficiently in advance of the adverse position, so as to prevent the cutting mechanism from digging into the ground. Although known header height controls have height setpoint adjusting means for selectively setting the header to a predetermined height above the ground, such known header height controls require that the operator stop the combine and climb down from the operator's platform in order to change the header height setting.
Known header height controls have a "deadband" within which the control is disabled when header height is between the raise and lower deadband limits at which the height sensors actuate the electrohydraulic power means to respectively raise and lower the header, and some controls also have means to vary the width of such deadband. However, known header height controls require that the operator stop the combine and climb down from the cab in order to vary the deadband. Further, it is difficult to adjust both deadband and height to obtain optimum settings in known header height controls.
For example, in one prior art header height control a four-bar mechanical linkage including a turnbuckle actuates a rotatable cam which operates raise and lower limit switches that derive electrical command signals for raise and lower solenoids. The header height setpoint represents the midpoint in the travel of the rotatable cam between the raise deadband limit at which the cam operates the raise limit switch and the lower deadband limit at which it operates the lower limit switch, and header height setpoint is adjusted by the operator getting out of the cab and changing the length of the turnbuckle in the four-bar mechanical linkage and similarly deadband is adjusted by changing the hole in a radial link of the four-bar linkage in which the turnbuckle is engaged.
Prior art header height controls have hysteresis at the raise deadband limit between the operate point and the release point for the raise power means and also have hysteresis at the lower deadband limit between the operate point and the release point for the lower power means, but the amount of hysteresis is known header height controls is not fixed but rather varies with both the height setpoint adjustment and with the deadband adjustment.
Known header height controls have both an automatic mode of operation and a manual mode which overrides the automatic mode, but known controls are unnecessarily complex in that they require a manual-automatic switch for placing the control in the manual mode and for restoring it to the automatic mode, as well as separate switches for raising and lowering the header when the control is in the manual mode. Such known header height controls require that the combine operator manipulate both the manual-automatic switch and the raise switch at the end of each crop row when he wants to lock out automatic operation so that the header will remain up, and also require that the operator manipulate the manual-automatic switch at the beginning of the succeeding crop row when he wants to restore the control to the automatic mode. Further, known electrical header height controls are sensitive to contamination and subject to faulty operation in utilizing electrical switches with separable contacts to initiate the raise and the lower power operations, and such separable switch contacts may undesirably be held open by particles of dust or dirt.